Everyone knows that the occupation of Iraq ended at 10.26am, June 28, 2004. In a hastily convened ceremony, US proconsul L Paul Bremer III swore in the interim prime minister, Iyad Allawi, and beat a rapid exit out of Baghdad's Green Zone. Soon after, Condoleezza Rice passed President Bush a note alerting him that Iraq had become a sovereign country. For posterity, the president scribbled his thoughts: "Let freedom reign!" And so it was. Barely a year after the invasion of Iraq, the occupation was over. Except for the 161,000 US and allied troops that remained.

If those forces seemed like a rather large caveat to the reigning of freedom, Bush had already resolved the dilemma: Iraqis would understand that "full sovereignty" - as the president termed his plan the previous month in an Army War College speech - was perfectly commensurate with the continued foreign presence. Purported nationalists in the insurgency would either embrace the logic and lay down their weapons or be wiped out at the hands of the increasingly powerful Iraqi Army. Anyone fighting the US would now be "fighting against an Iraqi government and Iraqis themselves," explained Richard Armitage, the deputy secretary of state. Since that was a senseless fight, everything would redound to the benefit of sovereign Iraq.

Three years later, it's safe to say things didn't turn out the way they were supposed to. Even so, according to a piece in the Washington Post by ace defence reporter Tom Ricks, the US military strategists are dusting off the old concept of a "post-occupation" foreign troop presence. This time, the thinking isn't that the post-occupation occupation will improve matters - the thought is just that it will stop them from becoming apocalyptic. Despite the admirable impulse, the fact remains that occupation is as zero-sum as pregnancy.

According to Ricks, the post-occupation plan under consideration envisions significant troop cuts by the time the Bush presidency ends in 2009: the occupation force of 150,000 troops would become the post-occupation force of at least 40,000. The post-occupation mission would be to guarantee the survival of the Iraqi government, combat al-Qaida, and mentor the Iraqi army and police. But the size of the force is less important than the message it sends to the Iraqis. As Ricks writes, "A reduction of troops, some officials argue, would demonstrate to anti-American factions that the occupation will not last forever while reassuring Iraqi allies that the United States does not intend to abandon the country."

That, however, is the exact same message of June 2004, which failed to reassure anyone. The US's allies in Iraq want the US to stay in force - if not forever, at least for some extended duration. The US's enemies in Iraq - a far more numerous and politically salient force - want the US out expeditiously. Anything that reassures one horrifies the other, leaving US troops caught in the crossfire. The Iraqi political process is meant to provide equilibrium for the complex dynamic of post-occupation, but it has only dragged the country into a zero-sum sectarian contest, with each side inspecting the US's intentions to see which faction it will back.

As the 2004 handover demonstrated, Iraqis are unlikely to be fooled into thinking 40,000-plus US forces stationed indefinitely in the country represents an end to the US presence. Worse, if the idea is to either protect Iraqis from a slide into chaos or safeguard enduring US interests - be it preventing genocide or fighting al-Qaida or keeping the oil flowing - then keeping only 40,000 troops in Iraq is senseless. As Major General Joseph Fil commented to Ricks: "My nightmare - the thing that keeps me up at night - is a failure of Iraqi security forces, somehow, catastrophically, mixed with a major Samarra mosque-type catastrophe." Leaving the Iraqi security forces aside, another huge sectarian provocation is guaranteed. In 2009, US commanders of a post-occupation force will find themselves powerless to deal with it. At that point, US troops will be little more than a constabulary force to keep the Iraqi politicians who failed to avert the crisis - and probably contributed to it - alive.

The maddening thing about a post-occupation is that the concept is thoroughly prudential. Who wouldn't want to hedge a bet for withdrawal, considering how awful the consequences of one could be? Unfortunately, if the consequences are really that bad, 40,000 troops won't be able to handle them, and the political pressure to reinforce them will be great. That, in a microcosm, is what allowed Bush to push the surge through, even as his party lost congressional power in a referendum on Iraq. That will leave two choices: reoccupation or withdrawal. Better to strategise around those choices - thoroughly - than convince ourselves that something called a "post-occupation" exists. We didn't learn that in 2004, and look where it got us.